Method for manufacturing a semicondutor component, as well as a semicondutor component, in particular a diaphragm sensor

ABSTRACT

A method for producing a micromechanical diaphragm sensor includes providing a semiconductor substrate having a first region, a diaphragm, and a cavity that is located at least partially below the diaphragm. Above at least one part of the first region, a second region is generated in or on the surface of the semiconductor substrate, with at least one part of the second region being provided as crosspieces. The diaphragm is formed by a deposited sealing layer, and includes at least a part of the crosspieces.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing asemiconductor component.

BACKGROUND INFORMATION

Semiconductor elements, and diaphragm sensors in particular, are knownalready, as well as methods for producing diaphragm sensors on the baseof semiconductor substrates such as silicon wafers. Flat porousdiaphragm regions are arranged on the semiconductor substrate assubstrate layer for sensor structures, for instance, and a cavity isproduced underneath the diaphragm for, e.g., the thermal insulation ofthe diaphragm.

The diaphragm sensors currently on the market are mostly fashioned asthin film diaphragm sensors. For this purpose, layer systems inthicknesses between a few 10 nm and a few μm are deposited on a carriersubstrate and after that, the carrier substrate is removed in predefinedareas, so as to obtain self-supporting areas. The structural elements ofthe sensor can then be arranged in the center of the diaphragm. Surfacemicromechanics (SMM), in which a sacrificial layer is generally usedthat is deposited on the front side of a carrier substrate beforediaphragm deposition, constitute another possibility for exposing thediaphragm. The sacrificial layer is later removed from the componentside of the sensor through “detachment openings” in the diaphragm,whereby a self-supporting structure is created. These surfacemicromechanical methods are comparatively costly, on account of thenecessity for separate sacrificial layers.

Published German patent document DE 100 32 579 discloses a method formanufacturing a semiconductor element and also a semiconductor elementproduced according to the method, in which, e.g., for a diaphragmsensor, a layer of semiconductor substrate material that was renderedporous is situated above a cavity. Two layers having different porosityare formed to produce the cavity using appropriate etching parameters.Whereas the first layer has lower porosity and seals up during asubsequent first annealing step, the porosity of the second layerincreases during the annealing step in such a way that a cavity or acavern is formed. In a second process step, at a higher annealingtemperature, a relatively thick epitaxial layer as second diaphragmlayer is grown on top of the first diaphragm layer formed from the firstporous layer.

It may also be provided that a thin epitaxial layer be grown during thefirst annealing step so as to ensure complete sealing of the porousfirst layer, which is used as starting layer for the epitaxy growth ofthe thick epitaxial layer. In this context, a lower growth rate ispreferred at a lower temperature compared to the subsequent depositionof the thick epitaxial layer.

As a result of the measures mentioned, the construction of an SMMsemiconductor element can be simplified considerably since noadditionally deposited sacrificial layer is required and, furthermore,the diaphragm itself or an essential portion of the diaphragm isproduced from semiconductor substrate material.

However, tests have shown that at least partially porous diaphragm mayget damaged already during production, or that damage may not always bereliably prevented under normal operating conditions. To avoid damage tothe diaphragm during manufacture or in regularly occurring applicationcases, published German patent document DE 101 38 759 provides a methodfor manufacturing a semiconductor element having a semiconductorsubstrate, in which the semiconductor substrate in the region of theporous diaphragm layer receives a doping that differs from the doping inthe region of the later cavity. After doping, the semiconductor materialof the diaphragm layer is rendered porous, and the semiconductormaterial underneath the semiconductor material that has been renderedporous is at least partially removed or rearranged to provide a cavity.

Published German patent document DE 100 30 352 discloses amicromechanical component which has a support body made of silicon and aregionally self-supporting diaphragm which is joined to the supportbody. The diaphragm is regionally and superficially provided with atleast one stabilizing element for support. To form the regionallyself-supporting diaphragm, it is provided that the silicon is renderedporous in a first region and is selectively removed via an etchingopening once the diaphragm layer has been deposited.

SUMMARY OF THE INVENTION

The present invention provides a method for producing a semiconductorcomponent, especially a micromechanical diaphragm sensor, as well as asemiconductor component having a semiconductor, a diaphragm and acavity. In this context it is provided that the semiconductor substratehas a first region having a first doping. It is also provided that thediaphragm is situated in the semiconductor substrate above the cavity.Various process steps are provided for producing the diaphragm. In afirst process step, on the surface of the semiconductor substrate, aboveat least one part of the first region, a second region is generatedhaving a second doping, it being especially provided that the seconddoping occupies different planes, that are distinguishable locally onthe surface of the semiconductor substrate, which, however, areconnected to one another, and consequently form a linked secomd region.It is advantageous if at least one part of the second region isdeveloped as crosspieces. In a second process step, the first region isat least partially dissolved out by an appropriate process control,e.g., by anodizing or by electropolishing. For the final production ofthe diaphragm, in a third process step, a closing layer is applied insuch a way that the material of this layer grows onto the crosspieces.Now, an important aspect of the present invention is that the closinglayer forming the diaphragm grows in a lateral and/or verticaldirection, starting from the growth on at least one part of thecrosspieces, and thus covers the first region. It may also be providedthat the cavity is generated from at least one part of the first region,the cavity being able to be generated before or after the third step.Besides the monocrystalline epitaxial layer, the diaphragm has at leastone part of the crosspieces.

Advantageously, the cavity in the first region is generated by a prebakemethod or a trench process.

In a further example embodiment of the present invention, thecrosspieces or a grating formed from the crosspieces is generated abovethe first region or at least partially in the first region.Advantageously, it is additionally provided that the crosspieces shouldbe generated in such a way that they are at least partially adjacent tothe first region. Advantageously, at least one part of the second regionis provided as a framing of the first region. In this context, theframing refers to the surface of the semiconductor substrate or to thevertical extension of the first region into the depth of thesemiconductor substrate, starting from the surface.

In one example embodiment of the present invention, the first region andthe crosspieces have different vertical extensions starting from thesurface into the depth of the semiconductor substrate. In this context,it is especially provided that the first region has a greater verticalextension compared to the crosspieces. Thereby, the crosspieces, withthe exception of the surface, are able to be enclosed on all sides bythe first region. In another embodiment of the present invention, it maybe provided that the crosspieces have a lesser vertical extension thanthe framing. The vertical and/or the lateral extension of thecrosspieces may vary too, in this context. In one embodiment of thepresent invention, it is provided that the crosspieces run right up tothe edge, i.e., up to the framing, the crosspieces and the framing beingn doped.

The elimination of the semiconductor material from the first region isachieved in one example embodiment of the present invention by amicromechanical process. In this context, by a suitable control of thismicromechanical process, a rendering porous of the first region may beachieved, a nearly uniform porosity of the first region being able to bebrought about. Furthermore, in the case of a suitable selection ofparameters which control the micromechanical process, a porosity of upto 100 percent may be achieved in the first region. In this context, aporosity of 100 percent corresponds to a cavern formation or a cavityformation. Micromechanical processes having such an effect are, forexample, electropolishing or an anodizing process.

In another example embodiment, before the depositing of the closinglayer onto the crosspieces, the porous semiconductor material and thecavity walls are oxidized chemically or thermally, for example. Thisgives the advantage that the epitaxy material is deposited on thecrosspieces, and not on the porous first region or the cavity walls.Thus, it may be prevented that the cavity grows through the material ofthe closing layer. The embodiment may also be used to lock in the oxideunderneath a diaphragm.

The cavity below the diaphragm made of the closing layer and thecrosspieces may be produced using different methods. Thus, there is apossibility of annealing the semiconductor substrate, after thedepositing of the closing layer, which represents a closed diaphragmlayer, and thus to induce the porous semiconductor material in the firstregion to rearrange itself. In response to a suitable selection of theannealing temperature, on account of surface effects, the diameters ofthe pores in the first layer increase, since the semiconductor materialcollects at the edges of the first region. In a further exampleembodiment of the present invention, the porous semiconductor materialis at least partially selectively eliminated by an etching process. Inthis context, it is provided that the selective elimination of theporous semiconductor material is achieved through at least one accesshole, which may be formed either before generating the first region orafter producing the closed closing layer in the semiconductor substrate.

In one further example embodiment of the present invention, the closinglayer is applied to the component side of the semiconductor substrate.For the purpose of eliminating semiconductor material of the firstregion, the access hole may optionally be generated in the semiconductorsubstrate from the back or the component side. If the access hole isclosed pressure-tight after elimination of the semiconductor material,one should take care that the pressure prevailing during the closingdefines the reference pressure in the cavity. Advantageously, the accesshole is put in the first region from the backside of the semiconductorsubstrate, and, after elimination of the semiconductor material, it isleft open. Thereby, in a simple manner compared to those of the priorart, one may generate a backside sensor or a differential pressuresensor having an exactly defined diaphragm thickness.

In one additional example embodiment of the present invention, theporous part of the first region and/or the cavity is passivated beforedepositing the closing layer. By this passivation, advantageously, adepositing of the material of the closing layer onto the semiconductormaterial and/or onto the cavity wall is prevented.

In one example embodiment of the present invention, in a further processstep, the diaphragm is structured in order to produce resonatorstructures. With the aid of such resonator structures, using the presentinvention acceleration sensors or yaw rate sensors may be generated, forexample. However, it is also conceivable to mount resistor structures onthe diaphragm which are able to record the motion of the diaphragm withthe aid of the piezoelectric effect.

In one further example embodiment of the present invention, the surfaceof the semiconductor substrate or the first region is irradiated using aradiation source. The regions thus irradiated subsequently generate thesecond region which, in one embodiment of the present invention, has thesame doping as the first region. By the radiation it is advantageouslyprevented that the semiconductor material, of the regions thusirradiated, is eliminated during the second step. The elimination of thesemiconductor material may, in this context, be brought about, forinstance, by porous etching or electropolishing. As radiation sources,light sources such as a laser are available. If a laser having amatchable wavelength is used, the possibility exists of selectingdifferent penetration depths of the irradiation or illumination, andthus to vary the formation of the second region or the verticalextension of the crosspieces, particularly during production. Methodsfor patterning the illuminated region on the surface of thesemiconductor substrate include shadow masks, diffraction patterns or(holographic) grids.

Besides the example embodiments of the present invention described sofar, the first and the second region may optionally have differentdoping. For example, the semiconductor substrate or the future cavityregion in the semiconductor substrate may have a p doping, and thecrosspieces may have an n doping. Furthermore, within the second region,a difference in the doping concentration may be provided for thecrosspieces and for the framing. Advantageously, the semiconductorsubstrate has silicon or a silicon compound as the semiconductormaterial. The first region of the semiconductor material in thesemiconductor substrate is rendered porous in such a way that a uniform,high porosity is created. In one embodiment of the present invention, itis provided that the first region under the crosspieces is at leastpartially rendered porous, and for the first region under thecrosspieces a vertical extension is provided that is twice as high asthe lateral extension of the crosspieces. In a further embodiment of thepresent invention, it is provided that the crosspieces are positioned ina lattice above the first region or above the cavity, between thecrosspieces on the surface access to the first region being possible.The holes that are created by the interspace between the crosspiecesare, in this context, provided in an example embodiment of the presentinvention to be smaller than 3 μm. In general, the closing layer may bedeposited by an epitaxial method. Moreover, between the first and thesecond region, a non-conductive boundary layer may be generated. Afterthe thermal oxidation and before the epitaxy, the oxidized, poroussemiconductor material may be etched out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d show various stages of a known method for manufacturing amicromechanical diaphragm.

FIGS. 2 a-2 b show the various stages of an exemplary embodiment of themethod of the present invention.

FIGS. 3 a-3 b illustrate the steps of a treatment of the surface toprevent the deposition of epitaxial material according to the presentinvention.

FIGS. 4 a-4 b show the steps of an additional exemplary embodiment forthe manufacturing of a diaphragm according to the present invention.

FIGS. 5 a-5 f illustrate various example embodiments of the crosspieceson which the diaphragm is deposited according to the present invention.

FIGS. 6 a-6 c show the steps of an example embodiment of a method forproducing crosspieces or lattices above a cavity.

FIGS. 7 a-7 c show the steps of another example embodiment of a methodfor producing crosspieces or lattices.

FIGS. 8 a-8 b show the steps of another example embodiment of a methodfor producing crosspieces or lattices.

FIGS. 9 a-9 b show the steps of another example embodiment of a methodfor producing crosspieces or lattices.

FIGS. 10 a-10 c show the steps of another example embodiment of a methodfor producing crosspieces or lattices.

FIGS. 11 a-11 b and 12 a-12 c show example embodiments of variouslattice and hole geometries according to the present invention.

FIGS. 13 a-13 g show various example embodiments of a method forproducing a lattice having a desired cross-section profile.

FIGS. 14 a-14 c show the steps of an example embodiment of a method forproducing a lattice.

FIGS. 15 a-15 d show the steps of an example embodiment of a method forproducing a lattice and/or a diaphragm.

FIGS. 16 a-16 c show the steps of an example embodiment of a method forproducing a lattice and/or a diaphragm.

FIGS. 17 a-17 d show the steps of an example embodiment of a method forproducing a lattice and/or a diaphragm.

DETAILED DESCRIPTION

As shown in FIG. 1 a, a conventional method for producing a diaphragmprovides using a porous double layer in a semiconductor substrate 100,i.e., a low-porosity layer 110 and a high-porosity layer 120 situatedunderneath it, it is possible to produce a starting layer 160 that issuitable for the growth of epitaxial layer 140, as well as a cavity 150(cf. FIG. 1 b). The transformation is accomplished by a first annealingstep (at approximately 900 to 1000° C.), during which the semiconductoratoms of low-porosity layer 110 rearrange in such a way that the surfaceseals. In the same or in a following annealing step, cavity 150 is thenable to be formed as well by rearrangement of the semiconductor atomsout of high-porosity layer 120. This is accomplished in that the poresenlarge under the influence of the annealing step and finally unite toform a “giant pore,” which then constitutes the cavity. To produce thediaphragm, an epitaxial layer 140 may then be deposited on startinglayer 160, which determine the characteristics of the diaphragm.However, there will be some pores that have not sealed in the productionof such a diaphragm since the available silicon is insufficient. In someareas several small pores also unite to form a large pore so that poreshaving diameters of up to 0.5 μm are created. The epitaxial layer, whichforms the later diaphragm, is unable to grow on these unsealed areas inthe starting layer in a monocrystalline manner, since the crystalstructure of the base is faulty. As a result, crystal faults form in theepitaxial layer (for instance, due to stacking faults). In the furthercourse of the process, piezoresistive resistors, for instance, which arerequired for the functioning of a pressure sensor, are produced on theupper surface of the diaphragm. Crystal faults in the epitaxial layermay degrade these resistors over the service life of the sensor andresult in drift of the sensor signal.

When producing a monocrystalline epitaxial layer, the deposition of afirst thin epitaxial layer (thinner than 1 μm, preferably 200 to 600 nm)may remedy this situation, in which additional semiconductor material isoffered for the complete sealing of the starting layer immediately atthe outset, during or at the end of the first annealing process. It maybe the same semiconductor material which is already present in layer 110and/or is used in epitaxial layer 140 that is deposited. Ifsemiconductor substrate 100 is made, for example, of silicon, such amethod gives the offered silicon atoms sufficient time to arrangethemselves according to the silicon crystal of the base. A lowertemperature and a reduced growth speed are advantageously selectedduring the growing of the thin epitaxial layer compared to thesubsequent, thick epitaxial layer. As an example, the temperature duringthe deposition of the thin epitaxial layer is approximately 900 to 1000°C., whereas the growth speed is selected to be less than 0.5 μm/min.

After the growth of this thin epitaxial layer and a possiblecontinuation of the annealing process, the substrate is exposed to ahigher temperature (preferably 1100 to 1200° C.) so that a second, thickepitaxial layer 140 may then be grown there (a few μm). Prior to thegrowing of the second epitaxial layer, further annealing at the highertemperature may also be carried out in order to heal any possiblecrystal faults in the sealed starting layer. Because of its relativelygreat thickness—compared to the starting layer and the first epitaxiallayer—the second epitaxial layer forms the actual diaphragm. Thisdiaphragm may then be used for a pressure sensor, for instance, or,following further patterning, for an acceleration sensor as well.

Depositing a single-crystalline, micromechanical diaphragm on a porousstarting layer has various weak points, which may have a negative effecton the production or the service life of the diaphragm. For example,cracks may form in the porous starting layer, which spread to theepitaxial layer deposited thereon. Furthermore, transitions from, forinstance, p⁺ doped starting layer 160 to a framing of the starting layermay occur. In an overlap of both dopings, the p⁺ doping is reduced to ap doping (see 130, FIG. 1 d). However, a lower p doping also causeshigher porosity. This effect is also utilized in the production of thedouble layer according to FIG. 1 a where low-porosity layer 110 has a p⁺doping, for instance, and high-porosity layer 120 has a lower p doping.Consequently, the low-porosity layer is connected to the edge via ahigh-porosity, and therefore fragile, region. Among others, cracks mayform here during the production.

To produce a monocrystalline diaphragm according to the presentinvention, as shown in FIG. 2 a, a first region 220 and a first portionof a second region 210 having different dopings are generated on asemiconductor substrate 200, according to the present method, firstregion 220 being also able to be composed of the substrate doping. Inthe following exemplary embodiment, it is to be assumed thatsemiconductor substrate 200 has a p doping, first region 220 a p or p⁺doping, and first and second portions 210 and 230 of the second regionhave an n or n⁺ doping. In this context, in a special exemplaryembodiment, the first portion of the second region that is denoted byreference numeral 210 in FIG. 2 a, is formed on the surface of thesemiconductor substrate as a type of crosspiece or lattice, whereas thesecond portion of the second region, that is denoted by referencenumeral 230 in FIG. 2 a, is formed as framing of first region 220. Inanother example embodiment of the present invention, it may be providedthat portion 230 and portion 210 of the first region have differentdopings, such as n⁺ or n.

Using a suitable etching process, for instance anodizing orelectropolishing, first region 220 is etched to be rendered porous. Inthis context, as already described, first region 220 may be both aportion of untreated semiconductor substrate 200 and an additionallydoped region. The latter has the advantage of allowing a sharperdelimitation in the production of porous region 220. Whereas firstregion 220 is etched porous up to a depth of 5 to 20 μm, for instance,starting from the surface of semiconductor substrate 200, second region210 is essentially not changed by the etching process. In an appropriateselection of the lateral extension of second region 210 on the surfaceof semiconductor substrate 200, first region 220 may be etched to berendered porous underneath second region 210 as well. For furtherprocessing, an exemplary embodiment provides that first region 220 havehigh porosity. Pore sizes may be of 1 nm up to several μm in diameter.As an alternative, high porosity may be achieved by producing very manysmall pores (˜5 nm) or a few very large pores (e.g., up to several 100nm). As in the production of large pores, the result is such that a lotof silicon is etched out. If semiconductor substrate 200 is made ofsilicon, a (natural) oxide layer can be found on the surface of theuntreated semiconductor substrate. For this reason, a reduction of the(natural) oxide layer on the silicon surface is obtained in a followingprocess step in that a short annealing of the semiconductor substrate orthe silicon substrate is carried out in a hydrogen atmosphere. As analternative, an “HF load” method or an HF GPE (gas phase etching) methodwith subsequent low-temperature epitaxy may be used to achieve the sameeffect. Afterwards, the semiconductor substrate is heated to growthtemperature (1000 to 1200° C.) before the deposition of amonocrystalline epitaxial layer 240 is initiated. The epitaxial layergrows predominantly on second regions 210 and 230. In an exemplaryembodiment, regions 210 are formed as monocrystalline crosspieces topromote the growth of the epitaxial material. The growth not only occursvertically, i.e., perpendicular to the surface, but laterally as well,so that the regions between individual crosspieces 210 are sealed by amonocrystalline layer. FIG. 2 b shows the diaphragm after epitaxiallayer 240 has been grown. During the growth process and under theinfluence of the growth temperature or during an additional annealingoperation, the highly porous silicon has been rearranged to form a largecavity 250. The form resulting from the growth of the free regionsbetween the crosspieces can be seen on the underside of the diaphragm inFIG. 2 b. Further annealing allows this form to fuse in a planar manner.

The highly porous silicon between monocrystalline crosspieces 210 isadvantageous here since it prevents the growth of silicon inside thecavity region. If the cavity were completely free, depending on thegrowth conditions, silicon could grow on the cavity walls from thebeginning of the epitaxial growth onwards. However, due to the initiallyporous surface between crosspieces 210, which rearranges itself over thecourse of the epitaxy process, it is prevented that silicon growsbetween crosspieces 210 or that a significant input of the epitaxymaterial into cavity 250 takes place.

Of course, the generation of an epitaxial layer having a cavity situatedunderneath it by means of being rendered porous, as it is described inthe present invention, may also be implemented when other materials orsemiconductor materials are used, and is not restricted to the use ofsilicon. However, this requires that these other materials orsemiconductor materials are also able to be rendered porous.

In another exemplary embodiment of the present invention, the regionbetween and underneath the crosspieces is not etched porous, but iscompletely eliminated. This may be accomplished by electropolishing, forinstance, and the porosity is increased by varying the etchingparameters (such as increasing the current, reducing the HFconcentration), to such a degree that it achieves 100 percent. Asdescribed earlier, the crosspieces are now able to grow together(closed), which entails the danger that within the cavity silicon alsogrows on. To avoid this, the inside of the cavity may be protected by alayer that prevents silicon from growing on there.

Such a protection is able to be achieved, for instance, by theapplication of a silicon nitride layer (such as Si₃N₄) 360 on thesurface of second regions 310, as shown in FIG. 3 a. Usingelectropolishing, second regions 310 which are in the form ofcrosspieces and are connected to one another and to framing 330, maysubsequently be exposed by removal of the first region (cf. crosssection through the semiconductor substrate in FIG. 3 a). An oxide 370is generated on the exposed regions of future cavity 350 via thermaloxidation, for instance. Using hot phosphoric acid, for example, nitride360 is subsequently able to be selectively dissolved away with respectto oxide 370. Such preparation allows a selective epitaxy to beachieved, i.e., the epitaxy material 340 (such as silicon) will growonly on the regions that are not protected by oxide 370. This preventssilicon growth in cavity 350.

Another possibility for selectively preventing the growth of silicon isto apply a thin (<60 nm) silicon oxide layer (SiO₂) to those regionsthat are not to be electropolished, prior to the application of siliconnitride layer (such as Si₃N₄) 360. After crosspieces 310 have beenexposed by a suitable etching process, for instance by electropolishing,an oxide may be generated on the exposed regions by thermal oxidation,as in the already discussed example. However, it must be taken intoaccount that this oxide has to be thicker than the oxide underneath theSiN mask. Subsequently, the nitride may be selectively dissolved withrespect to the oxide. As an alternative, a dry-etching process may becarried out as well in which the oxide and the nitride have the sameetching rates. By suitable selection of the etching time it is possibleto ensure that a sufficiently thick oxide layer remains on the cavitywalls once the oxide that was situated underneath the SiN has beendissolved. Finally, a selective epitaxy may then be carried outanalogously to the above exemplary embodiment.

In yet another exemplary embodiment, in which the semiconductorsubstrate is made of silicon, for instance, the silicon is renderedporous by etching between and underneath the crosspieces. Afterwards, athin layer of oxide, which is a few nm thick, for example, is generatedon all silicon surfaces (i.e., on the wafer surface as well as on thesurface of the pore walls). This may be done, for instance, by thermaloxidation, by oxygen-plasma treatment, by chemical oxidation or alsosome other type of treatment. This oxide prevents the rearrangement ofthe porous silicon during additional high-temperature steps as arerequired in the epitaxy or other furnace processes. The oxide on thewafer surface may now be removed by a brief HF dip using dilutedhydrofluoric acid (HF), or a dry chemical oxide etching process usingClF₃ or XeF₂ Due to the surface tension, the hydrofluoric acid does notpenetrate the pores, so that the oxide layer continues to remain on thepore walls. A corresponding description of such a pretreatment forstabilizing the porous silicon for a subsequent epitaxial growth can befound, for instance, in the article “Low-pressure vapor-phase epitaxy ofsilicon on porous silicon,” Material Letters 94 (1988), by L. Vescan etal. An expitaxial layer may subsequently be grown, this layer growingpredominantly on the monocrystalline crosspieces. The surface is sealedby lateral growth. After the epitaxy, an access hole may be etchedthrough the epitaxial layer from the front side, for instance, or alsofrom the back side of the semiconductor substrate, and the oxidizedporous silicon be selectively dissolved out through the access hole. Asan option, the access hole may also be sealed again after the removalprocess, it being taken into account that the pressure prevailing duringsealing of the access hole defines the reference pressure in the cavity.

The diaphragm produced in this manner may be used, for instance, for apressure sensor having piezoresistive resistors. For this purpose, acircuit may be integrated adjacent to or on top of the diaphragm. If anaccess hole for the selective removal of the semiconductor material inthe second region was generated on the backside, a backside sensor ordifferential-pressure sensor is obtained, which has a preciselyspecified diaphragm thickness compared to the related art. If an accesshole is made from the front side, this hole must be sealed again in apressure-tight manner for the pressure-sensor application.

In another exemplary embodiment, it is provided to pattern the diaphragmin the form of resonator structures. The use of such structures makes ispossible to implement acceleration sensors and/or yaw-rate sensors, forinstance.

One possibility for stabilizing the crosspieces before the epitaxiallayer is grown is to optionally generate supports underneath thecrosspieces, which melt away during subsequent high-temperature steps(epitaxy or annealing steps (oxidation, diffusion) for the generation ofintegrated circuits) due to rearrangement of the silicon atoms. As canbe seen from FIG. 4 a, in region 460, columns are created if crosspieces410 are wider than half of the etching-depth in first region 420. Theisotropic undercut-type etching during anodization is then insufficientto anodize the silicon underneath the broadened crosspieces 410 or todissolve it out. For reasons of energy (minimizing the surface energy)silicon rearranges itself at high temperatures (>1000° C.). In theprocess, the column (region 470) “melts” and the firm connection betweensubstrate and diaphragm is broken. As a result, the diaphragm can movefreely, as shown in FIG. 4 b.

Instead of individual crosspieces, the second region above the cavitymay also be arranged in the form of a lattice, porous regions 510 andcrosspieces 500 alternating with each other. In FIGS. 5 a to 5 f,various possible example embodiments are shown, the mentioned examplesnot being intended to be considered as final, limiting illustrations. Inthis context, better resist adhesion is noticeable in FIGS. 5 e and 5 f,for example.

Various methods are available to implement the crosspieces or thelattice on which the monocrystalline diaphragm is deposited in a latermethod step. For instance, in one exemplary embodiment the crosspiecesor the lattice, instead of being generated by local n doping, aregenerated by local amorphization of the single-crystalline Si substrate.As shown in FIG. 6 a, the single-crystalline semiconductor substratemade of silicon 520 is bombarded with high-energy ions 540, such asargon ions. Due to this bombardment and the use of an implantation mask530, for instance made of SiO₂, the single-crystalline structure isdestroyed and regions 550 having amorphous silicon are created. Theamorphous Si 550 remaining behind is not attacked in the followinganodization in hydrofluoric acid, so that underneath amorphous regions550 a region 560 of porous Si is created (FIG. 6 b), which is able torearrange itself to form a cavity in a subsequent annealing process. Inthis way an amorphous Si lattice 550 is produced, which may be overgrownby an epitaxy process prior to or after this subseque nt annealingoperation. Corresponding to the amorphous base, epitaxial layer 590(FIG. 6 c) does not become single-crystalline, but polycrystalline, incontrast to layer 570 which grows on monocrystalline region 520. Thetransition between polycrystalline region 590 and monocrystalline region570 is determined by the epitaxial parameters.

In one variant of this exemplary embodiment, additional annealing may becarried out before the epitaxial layer is grown. Due to this annealing,the amorphous crosspieces are able to recrystalize and rearrange in theform of a single-crystalline lattice. This recrystallization step makesit possible to generate a monocrystalline Si epitaxy on the latticecrosspieces.

In another exemplary embodiment for producing an Si lattice on poroussilicon, the illumination selectivity of the anodization process may beutilized. In this context, as illustrated in FIGS. 7 a to 7 c, a p dopedsilicon substrate 700 is illuminated (using ions 710 or laser beam 735)during anodizing, so that regions 705 are created, which counteract theanodizing process due to the charge substrates generated by the internalphoto effect. With the aid of a suitable shadow mask 715, a diffractionpattern 720 or a holographic lattice (FIG. 7 c), a lattice-shaped region705 of substrate 700 to be anodized may be illuminated and therebyprotected from being rendered porous. Since the penetration depth of thelight is limited as a function of the wavelength, the region protectedin this manner is ultimately etched in an undercut manner. The followingepitaxial process or the generation of the cavity may then be carriedout in a manner that is analogous to the method already described.

To produce a holographic lattice, a laser beam 735, as shown in FIG. 7c, may be guided onto a beam splitter 730, the two partial beams beingreflected at mirrors 740 and 745 and interacting with one another on thesubstrate surface in region 705.

In an extension of the last exemplary embodiment, it may also beprovided to first produce an n doped Si epitaxial layer 755 on a p dopedSi substrate 750, as shown in FIG. 8 a. According to FIG. 8 b andfollowing the previous exemplary embodiment according to FIGS. 7 a to 7c, n-doped Si epitaxial layer 755 is illuminated using suitableillumination 760 and a shadow mask 770 (of metal, for instance). Theepitaxially generated n Si is etched non-porous without illumination,since no p holes (defect electrons) are present. The local illuminationproduced using mask 770 generates the required charge substrates in then-doped region, so that the n-doped epitaxial layer is able to berendered locally porous by etching at these locations 780. If theetching procedure reaches p doped substrate 750 lying underneath,undercut-type etching is performed across the entire surface. Suchundercut-type etching makes it possible to render substrate 750 porousby etching in region 765, since no illumination is required in the pdoped region. In addition to a shadow mask 770, diffraction patternsand/or holographic lattices as shown in FIGS. 7 b and 7 c, may be usedas well to produce local illumination on the surface of epitaxial layer755.

Another example for producing a lattice on porous silicon is the use ofa patterned, n doped Si epitaxyial layer. To generate this patterned ndoped Si epitaxial layer 805, an n doped Si epitaxial layer 805 isdeposited on a p doped Si substrate 800, as shown in FIG. 9 a, and theformer is still unpatterned at this stage. Subsequently, an oxide mask810 is deposited, which may be patterned by HF, for instance. In afurther process step, n doped Si epitaxial layer 805 may then bepatterned by means of trenches 815 via the trench mask generated inoxide 805. Since the trench process does not stop on p Si substrate 800,trenches 815 must be produced in a time-controlled manner. However,slight over-etching into substrate 800 is not critical. The structureobtained of oxide 810, n doped epitaxy 805 and p doped substrate 800 inthis manner is then rendered porous by etching by anodization in HF, asshown in FIG. 9 b. Oxide 810 and n doped epitaxial layer 805 are notattacked in this context, whereas p doped Si substrate 800 is renderedporous by etching. Prior to the subsequent epitaxial deposition of thediaphragm, oxide 810 is removed so that the diaphragm is able to grow onthe n doped lattice crosspieces.

In a further exemplary embodiment, as shown in FIGS. 10 a through 10 c,a lattice may be produced on porous silicon 845 in that an n doped Siepitaxial layer 840 is selectively grown on a patterned SiO₂ or Si₃N₄mask 835. In this context, SiO₂ or Si₃N₄ mask 835 acts in a passivatingmanner in such a way that monocrystalline silicon 840 is able to growonly on exposed Si substrate 830, that is, between oxide regions ornitride regions 835, as shown in FIG. 10 b. As already describedpreviously and shown in FIG. 10 c, it is subsequently possible, afterthe removal of oxide/nitride regions 835, to generate a porous region845 in substrate 830, which may be reshaped into a cavity in a followingannealing process.

An additional exemplary embodiment uses different porosities in thelattice region and in the cavity region. Such an adaptation of theporosities in the mentioned regions allows the optimization of therearrangement of the porous silicon into a cavity or the growth of theepitaxial silicon diaphragm. For instance, it may be provided that ahigher or lower porosity be generated in the cavity region than in theregion of the holes. In addition to a sharp separation of the differentporosity regions, a porosity gradient is also conceivable.

However, in the starting phase of the epitaxy for forming the diaphragm,attention must be paid that the cavity layer forms a sufficiently stablebase, so that an excessively high porosity in the cavity layer would bedisadvantageous. Furthermore, it would be desirable to produce highporosity in the holes in the lattice region as well, since this speedsup the formation of the holes at the beginning of the epitaxy growth. Byappropriate selection of a (high) porosity in the lattice area, it maythus be prevented that the growth begins on the rearranged silicon inthe lattice holes. For, if at least a portion of the epitaxial growthwere to begin on the silicon between the lattice crosspieces during therearrangement, this might cause crystal faults to be created, whichwould propogate in the diaphragm layer, for example as stack faults.

To optimize the mechanical properties of the epitaxial Si diaphragm, thelattice geometries and the hole geometries or the situation of the holesmay be varied locally, as is shown in FIGS. 11 a and 11 b by way ofexample. Such a local variation may achieve an improved edge fixing ofdiaphragm 855 on substrate 850. It is conceivable in this case thatlattice holes 860 could be omitted in a regular pattern, for instance atthe diaphragm's edge, as is illustrated in FIG. 11 a. In this example,every other lattice hole at the outermost edge of the hole geometry isomitted, so that no lattice hole 860 is to be found at location 865, forinstance. In addition, however, it may also be provided that theoutermost row of holes has a smaller diameter compared to the moreinwardly lying holes. Another alternative is to broaden the latticecrosspieces at the edge of the diaphragm. In addition, however, the(hole) lattice may also be arranged across only a portion of thediaphragm region, so that a self-supporting diaphragm is created asshown in FIG. 11 b. In this context, substrate 850 encloses the etchingregion, which in turn is covered by regions 875 having lattice holes 860and regions 870 not having lattice holes 860. The diaphragm may then beepitaxially deposited on regions 870 and 875 patterned in this manner.

The mechanical properties of the diaphragm, such as the resonantfrequency and/or the rigidity, may also be varied by correspondinglyadapted geometries of the lattice crosspieces and holes. One possibilityfor increasing the rigidity is to omit holes in the center of thediaphragm. However, reliable undercut-type etching of the holes must beensured. This requirement results in a limit for the maximum number ofholes that may be omitted.

If electrically active elements are applied on the diaphragm, it mayalso be useful to adapt the lattice geometry and hole geometry locally.For instance, piezoresistive resistors, such as for a pressure sensor,may be situated in a region that stands out as a result of an especiallydefect-free epitaxy (avoidance of leakage currents and shunts viaso-called diffusion pipes). This may be accomplished in that, anespecially low number of lattice holes, to be overgrown epitaxially, ispresent in these regions and/or that an especially adapted latticegeometry and hole geometry is used, which is overgrown in a particularsatisfactory manner.

Of course, it may also be provided to superpose a plurality of differentlattice geometries and lattice profiles and to combine them in thismanner. For instance, FIG. 12 a shows the simultaneous use of twodifferent rectangular geometries. A second monocrystalline lattice 890or 895 having broader or thicker crosspieces is superposed on top offirst monocrystalline lattice 885 having narrow crosspieces andincluding porous regions 880. Such a combination may make possible bothan additional local diaphragm reinforcement and a reinforced borderingof the diaphragm in substrate 850.

A schematic cross section through a diaphragm region having differentlattice geometries is shown in FIG. 12 b. It can be seen clearly thatcrosspieces 885 and 890 have different lateral extensions. Thesuperpositioning of various geometries in the form of a lattice may alsobe achieved by different implantations. In this context, variations ofthe crosspiece profiles of the lattice are likewise possible as isillustrated by the cross section shown in FIG. 12 c. In this example twodifferent crosspieces 885 and 895 were produced by differentimplantation energies and thus different penetration depth.

In addition to the arrangement of the holes in the diaphragm region, itis also possible, as already mentioned, to adapt the cross-sectionprofiles of the lattice to the requirements of the diaphragms to beproduced. In the least complicated case, as shown in FIGS. 13 a and 13b, implantation areas 905 (e.g., n doped regions in a p doped substrate)are introduced in substrate 900 to form a lattice 920. Masks 910 ofphotoresist or Si oxide, which are irradiated by means of animplantation method 915, are used for the selective patterning ofimplantation regions 905. In this implantation process, the energy orthe particles used in implantation method 915 may be adapted to thesubstrate. After removal of mask 910, substrate 900 may be renderedporous around implantation areas 905, so that crosspieces 920 arecreated in porous region 925.

Suitable selection of the structure of mask 910 (such as a grey-tonemask of photoresist or Si oxide) in conjunction with an appropriateimplantation method 915 allows the lattice profile to be influenced in avariety of ways as is shown in FIGS. 13 c and 13 d. The triangularcross-section form of implantation regions 905 illustrated in thesefigures has advantages with respect to the rearrangement of poroussilicon and the subsequent epitaxy. Here, the holes seal faster as aresult of the smaller hole diameter on the substrate surface. Moreover,the region that must be overgrown by the epitaxy is smaller. This leadsto fewer crystal faults in the epitaxial layer forming the diaphragm.

In addition to a simple implantation, a multiple implantation usingdifferent masks and/or implantation energies is conceivable as well. Apossible result of a duplicate implantation having an increase in theimplantation energy in the second implantation step (FIG. 13 f) is shownin FIGS. 13 e to 13 g. In FIG. 13 g, a similar lattice profile resultsas that which was already shown in a masking of the substrate accordingto FIG. 13 c.

In general, multiple implantations using appropriate masks and varyingimplantation energies are able to generate virtually any latticecross-section profiles.

FIGS. 14 a and 14 b illustrate another possibility for producing alattice on or within a region rendered porous by etching. In the process(shown in FIG. 14 a), an SiC layer 960 is deposited via CVD (silane &propane) on a p doped Si substrate 950 and patterned using an oxide mask970, for instance, in a wet method (in KOH, KClO₃ or the like) or a drymethod (for instance SF₆). As shown in FIG. 14 b, via holes 975 betweenlattice 965 created by the patterning of SiC layer 960, the p dopedsilicon may be etched porous by selective anodizing with respect to theSiC in region 980. The reason for this selective etching is that SiChaving 2.4 eV (indirect) or 5.3 eV (direct) has a markedly greater bandgap than Si. After lattice 965 has been produced, SiC layer 960 may beremoved from the surface of the semiconductor substrate down to lattice965, and a frame 990 which surrounds lattice 965. Silicon may then beepitaxially deposited on semiconductor substrate 950 or on lattice 965to form a diaphragm. In the process, a monocrystalline Si layer 955 isgrown on the edge of semiconductor substrate 950 and a polycrystallineSi layer 995 is grown on SiC 965 and 990, as shown in FIG. 14 c. Thetransition between monocrystalline region 955 and polycrystalline region995 is determined by the epitaxy parameters. Angle 999 isdeterminatively a function of these parameters. The porous region may betransformed into a cavity region via an annealing step prior to orfollowing the epitaxy operation.

In a further exemplary embodiment, the lattice and diaphragm productionmay be accomplished via an additional p⁺ doping. This additional p⁺doping is able to broaden and improve on the method described inpublished German patent document DE 101 38,759, for instance. Asillustrated in FIG. 15 a, a layer 1010 having an additional p⁺ doping isproduced on a p doped semiconductor substrate 1000, such as an Sisubstrate, before semiconductor substrate 1000 and layer 1010 arecovered by an n doped epitaxial layer 1020. Subsequently, n dopedepitaxial layer 1020 may be patterned, using an oxide mask 1030, forinstance via a trench process. Within the framework of the patterning itis provided that holes or trenches 1060 are created in epitaxial layer1020 via which an anodization process may be implemented to produce a(nano-)porous, p doped layer 1040 in semiconductor substrate 1000. Sincep⁺ doped layer 1010 is less susceptible with respect to this anodizingprocess, a layer 1050 which has meso-pores and is located above thelayer having nano-pores is produced in this region, and has lowerporosity than layer 1040. In a subsequent annealing process, thematerial in nano-porous layer 1040 rearranges itself to form a cavity,whereas the material in the meso-porous layer 1050 rearranges to form asealed layer. The formation of the sealed layer facilitates both thesealing and the overgrowing of lattice holes 1060 during the followingepitaxy and also the mechanical stability of the lattice duringannealing prior to the epitaxy.

A further advantage of using an additional p⁺ doping results from abetter adaptation of the anodization during production of the lattice.Without the p⁺ doping, the p doped substrate is etched beneath lattice1070 in an undercut manner in the form of a beak 1080, as illustrated inFIG. 15 c. This beak may be reduced or prevented by additional p⁺ doping1010, so that a considerably rounder shape is formed at the underside oflattice crosspieces 1070, as illustrated in FIG. 15 d.

It has been shown to be especially advantageous if, besidessemiconductor substrate 1000 of p doped silicon, p⁺ doped layer 1010 andn doped layer 1020 are also made of silicon. However, the use of othersemiconductor materials is conceivable as well.

Another exemplary embodiment for producing a lattice and/or a diaphragmon a semiconductor substrate relates to the production of p⁺ dopedregions in an n doped layer. In this context, a planar n doped(monocrystalline) layer 1110 is first deposited on a p doped substrate1100, as shown in FIG. 16 a. It is possible here that n doped layer 1110is introduced in p substrate 1100 by means of implantation or by coatingor thin epitaxy. Subsequently, p⁺ doped regions 1120 are introduced in ndoped layer 1100. This introduction is advantageously implemented by animplantation process in which the p⁺ doping must be sufficiently strongto locally redope n doped layer 1110. However, besides an implantationprocess, other methods that produce p⁺ doped regions 1120 within n dopedlayer 1110, may be utilized as well.

The structure thus produced and illustrated in FIG. 16 b maysubsequently be anodized, n doped layer 1110 not being attacked andremaining on substrate 1100 in monocrystalline form. In contrast, localp⁺ doping 1140 is etched to be rendered porous. If anodizing isimplemented for a sufficiently long period of time, not only p⁺ dopedregions 1140 but also region 1130 are etched porous as shown in FIG. 16c, region 1130 being located in p substrate 1100 underneath p⁺ dopedregion 1140. Therefore, by way of the n doped regions above region 1130,etched to be rendered porous, lattice crosspieces of n doped materialare obtained, between which p⁺ doped material is located, which isetched to be rendered porous.

In a further exemplary embodiment (shown in FIGS. 17 a through 17 d) forproducing a monocrystalline lattice on porous semiconductor material, ap doped substrate 1200 may first be patterned by means of a firstpatterning. This first patterning essentially defines the laterdiaphragm region. The first patterning is advantageously selected suchthat it has half of the period of the later lattice constant, i.e., theclearance of holes 1210 with respect to each other. An n doped epitaxiallayer 1220 is deposited on p doped substrate 1200 patterned in thisfashion. Naturally, it may also be provided that n doped layer 1220 isgenerated by n doping, e.g. by coating or diffusion, directly insubstrate 1200. A portion of n doped layer 1220 is subsequently removedby a physical etching step, so that a reduction in the lattice constantis achieved, as illustrated in FIG. 17 c by way of example. If thethickness of layer 1220 has been selected appropriately, the latticeconstant is able to be halved as a result.

Due to the lowering of the lattice constant, a considerably finerstructure of the lattice crosspieces or holes 1210 is obtained on thesurface of substrate 1200, so that it is easier for holes 1210 to becomeovergrown. An anodizing process follows, which once again does notattack the n doping, but renders the p doping of the substrate porous byetching, ultimately forming a region 1230, which etches the n dopedlattice crosspieces in an undercut manner (FIG. 17 d). As has beenmentioned several times, annealing and/or an epitaxy are/is carried outas final step in the production of the diaphragm. The annealingrearranges the porous semiconductor material in region 1230 and sealsthe holes between the lattice crosspieces. The actual diaphragm, on theother hand, is formed by the epitaxy.

Due to the manner in which the n doped lattice is produced in thisexemplary embodiment, only geometries that form cohesive latticecrosspieces after physical etching may be used, for instance, achessboard-like geometry (see, e.g., FIG. 5 e) or a rod-shaped lattice(see, e.g., FIG. 5 f).

Silicon is used as semiconductor material for the above-describedmanufacturing method for producing a lattice above a porous layer. Itshould be noted, however, that materials or semiconductor materialsother than silicon, which are able to be rendered porous by etching viaan electrochemical method, for instance, may also be used in themanufacturing method.

1. A method for producing a semiconductor component configured as amicromechanical diaphragm sensor, the semiconductor component includinga semiconductor substrate having a first region with a first doping, adiaphragm, and a cavity that is located at least partially below thediaphragm, the method comprising: providing a second region having asecond doping above at least one part of the first region, the secondregion being provided one of in and on the surface of the semiconductorsubstrate, wherein at least one part of the second region is configuredas crosspieces; at least partially dissolving out the semiconductormaterial in at least one part of the first region; depositing a sealinglayer above the second region, wherein the sealing layer seals thesurface above the first region, starting out from at least one part ofthe crosspieces of the second region, in at least one of lateraldirection and vertical direction; and providing the cavity from at leastone part of the first region; wherein the diaphragm is formed by thedeposited sealing layer, and wherein the diaphragm includes at least apart of the crosspieces.
 2. The method as recited in claim 1, wherein atleast one of: a) the crosspieces are generated one of above the firstregion and in the first region; and b) the crosspieces are at leastpartially adjacent to the first region.
 3. The method as recited inclaim 3, wherein at least a part of the second region is provided as aframing for the first region, wherein the framing and the crosspiecesdiffer in at least one of relative degrees of doping and relative dopingconcentrations.
 4. The method as recited in claim 1, wherein thevertical extension of the first region and the vertical extension of thecrosspieces differ, starting from the surface of the semiconductorsubstrate, and wherein at least one of: a) the first region has agreater vertical extension than the crosspieces; b) the crosspieces havea lesser vertical extension than the framing; and c) the crosspiecesvary in at least one of vertical and lateral extensions.
 5. The methodas recited in claim 1, wherein the dissolving out of the semiconductormaterial in the first region is achieved by a micromechanical process,wherein the micromechanical process is such that, in response to asuitable selection of parameters controlling the micromechanicalprocess, at least one of: a) the semiconductor material is renderedporous in at least one part of the first region, a porosity of up to100% being provided; and b) the cavity formation is achieved in thefirst region by one of an anodizing process and electropolishing.
 6. Themethod as recited in claim 5, wherein, before the depositing of thesealing layer, one of the porous semiconductor material and the cavitywalls are oxidized.
 7. The method as recited in claim 6, wherein thecavity is formed by one of: a) an annealing step after the depositing ofthe sealing layer, by rearrangement of the porous semiconductormaterial; and b) an at least partially selective dissolving out of theporous semiconductor material, wherein the selective dissolving out ofthe porous semiconductor material takes place through at least oneaccess hole.
 8. The method as recited in claim 7, wherein the sealinglayer is applied on the front side of the semiconductor substrate, andwherein the access hole is generated from one of the back side of thesemiconductor substrate and the front side of the semiconductorsubstrate, and wherein for the case of the access hole generated fromthe front side, the access hole is sealed after the dissolving out ofthe semiconductor material, in a pressure-tight manner.
 9. The method asrecited in claim 1, wherein at least one of a) the porous semiconductormaterial in the first region and b) the cavity is passivated before thedepositing of the sealing layer, and wherein the passivationsubstantially prevents a depositing of the sealing layer at least one ofon the porous layer and onto the cavity walls.
 10. The method as recitedin claim 1, further comprising: patterning the diaphragm to form avibratory structure.
 11. The method as recited in claim 1, whereinduring the step of at least partially dissolving out the semiconductormaterial in at least one part of the first region, the surface of thesemiconductor substrate is irradiated by a radiation source in theregion of the crosspieces, whereby the irradiation prevents a dissolvingout of the material in irradiated areas, the surface being irradiated atleast one of by a laser and with the aid of a patterning mask.
 12. Themethod as recited in claim 3, wherein at least one of: a) at least oneof: i) the first and the second regions have different dopings, ii) thesealing layer is deposited using an epitaxial method, and iii) thecrosspieces and the framing have different doping concentrations; b) atleast one of: i) the semiconductor substrate has silicon, ii) after thestep of at least partially dissolving out the semiconductor material inat least one part of the first region, the first region has a uniformlyhigh porosity, and iii) between the first and the second region, anon-conductive boundary layer is generated; c) at least one of: i) thesemiconductor material of the first region is at least partiallyrendered porous below the crosspieces, ii) the crosspieces are situatedin a lattice above one of the first region and the cavity, wherein holesbetween the lattice crosspieces have a diameter that is less than 3 μm,iii) after a thermal oxidation and before the epitaxy, the oxidizedporous semiconductor material is dissolved out, and iv) before thedeposit of the sealing layer, the porous semiconductor material isselectively removed by one of an additional wet-chemical etching usingone of TMAH and KOH, and an additional dry-chemical etching using one ofClF₃ and XeF₂.
 13. A semiconductor component configured as amicromechanical diaphragm sensor, comprising: a semiconductor substratehaving a first region of a first doping, wherein a cavity is provided inat least one part of the first region; a second region having a seconddoping distinguishable from the first doping, the second region beingabove at least one part of the first region, wherein at least one partof the second region is configured as crosspieces; and a diaphragm of amonocrystalline, epitaxial layer mounted on the crosspieces, wherein thecrosspieces are located above at least one part of the cavity.